Reciprocal quantum logic comparator for qubit readout

ABSTRACT

One aspect of the present invention includes a reciprocal quantum logic (RQL) readout system. The system includes an input stage on which a read pulse is provided and an output stage configured to propagate an output pulse. The system also includes an RQL comparator comprising a first Josephson junction and a second Josephson junction that are coupled to a qubit. A bias current switches between a first Josephson junction in a first quantum state of the qubit and a second Josephson junction in a second quantum state of the qubit. The first Josephson junction triggers to provide the output pulse on the output stage in the first quantum state in response to the read pulse and the second Josephson junction triggers to provide no output pulse on the output stage in the second quantum state in response to the read pulse.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 14/202,724, filed 10 Mar. 2014, which is incorporated herein in its entirety.

This invention was made with Government support under Contract No. W911NF-11-C-0069, Project No. 36545. The Government has certain rights in this invention.

TECHNICAL FIELD

The present invention relates generally to quantum and classical digital superconducting circuits, and specifically to a reciprocal quantum logic (RQL) comparator for qubit readout.

BACKGROUND

Superconducting digital technology has provided computing and/or communications resources that benefit from unprecedented high speed, low power dissipation, and low operating temperature. A typical goal in the implementation of a superconducting circuit is very high-speed manipulation of data (e.g., tens of Gigahertz) in a manner that is very power efficient and minimizes losses through temperature. Superconducting technology can implement qubits to store quantum information. One example of a qubit is a phase qubit, which can be configured as an LC resonator, such as formed from an inductor shunting a Josephson junction. A control circuit can be employed to write to the phase qubit and a readout circuit can be employed to read a quantum state from the phase qubit.

SUMMARY

One example includes a readout system configured to read a quantum state of a qubit. The system includes an input stage on which a read pulse is provided and an output stage. The system also includes a comparator circuit coupled to the qubit and configured to provide an output pulse on the output stage in response to the read pulse to indicate a first quantum state of the qubit, and to not provide the output pulse on the output stage in response to the read pulse to indicate a second quantum state of the qubit.

Another example includes a method for reading a quantum state of a phase qubit. The method includes providing a bias current in a first direction in response to a first quantum state of the phase qubit and in a second direction in response to a second quantum state of the phase qubit and applying a read pulse on an input stage. The method also includes providing an output pulse from a comparator circuit to an output stage in response to the read pulse based on the bias current being provided in the first direction. The method further includes not providing the output pulse from the comparator circuit to the output stage in response to the read pulse based on the bias current being provided in the second direction.

Another example includes an reciprocal quantum logic (RQL) readout system configured to read a quantum state of a qubit. The system includes an input stage on which a read pulse is provided in response to an RQL clock signal. The system also includes an output stage configured to propagate an output pulse in response to the RQL clock signal. The system further includes an RQL comparator coupled to the qubit and configured to provide an output pulse on the output stage based on triggering a first Josephson junction in response to the read pulse and the RQL clock signal to indicate a first quantum state of the qubit, and to not provide the output pulse on the output stage based on triggering a second Josephson junction in response to the read pulse and the RQL clock signal to indicate a second quantum state of the qubit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of an RQL readout system.

FIG. 2 illustrates an example of an RQL readout circuit.

FIG. 3 illustrates an example of a timing diagram.

FIG. 4 illustrates an example of a method for reading a quantum state of a phase qubit.

DETAILED DESCRIPTION

The present invention relates generally to quantum and classical digital superconducting circuits, and specifically to a reciprocal quantum logic (RQL) comparator for qubit readout. An RQL readout system can include an input stage on which a read pulse is provided. The input stage can include at least one input Josephson junction on which the read pulse can propagate. The read pulse can be provided, such as from an external RQL circuit, to determine a quantum state of a phase qubit. The RQL readout system can also include an output stage configured to propagate an output pulse in a first quantum state of the phase qubit in response to the read pulse. If the phase qubit is in the second quantum state, the output pulse is not provided on the output stage in response to the read pulse. The RQL readout system also includes an RQL comparator comprising a first Josephson junction and a second Josephson junction that are coupled to the phase qubit. The quantum state of the phase qubit can set a relative threshold associated with each of the first and second Josephson junctions. As an example, the phase qubit can be inductively coupled to the RQL comparator to provide a bias current flow in either a first direction or a second direction through the Josephson junctions depending on the quantum state of the phase qubit. Thus, one of the first and second Josephson junctions can trigger in response to the read pulse based on the bias current being either added to or subtracted from the read pulse based on the direction of current flow of the bias current through the first and second Josephson junctions, respectively, to either provide or not provide the output pulse on the output stage.

The RQL readout system also includes an RQL clock that provides RQL clock signal to each of the input stage, the output stage, and the RQL comparator. Because the phase qubit can be inductively coupled to the RQL comparator, the phase qubit can be effectively isolated from the RQL clock and the associated shunting resistors. As a result, the phase qubit is decoupled from potential noise sources to substantially prevent degradation of coherence time of the phase qubit. The RQL clock signal can affect the thresholds of the Josephson junctions in the input stage, the output stage, and the RQL comparator to facilitate triggering of the Josephson junctions, such that the read pulse can be provided at a first cycle (e.g., positive cycle) of the RQL clock signal. During a second cycle (e.g., negative cycle) of the RQL clock signal, a negative pulse can be provided to reset the respective Josephson junctions, thus allowing subsequent readout of the quantum state of the phase qubit.

FIG. 1 illustrates an example of an RQL readout system 10. The RQL readout system 10 can be implemented in a variety of quantum and classical computing environments to read a quantum state from a superconducting qubit 12. As an example, at least a portion of the RQL readout system 10 can be implemented on an integrated circuit (IC) in a superconducting environment. In the example of FIG. 1, the qubit 12 is configured as a phase qubit. For example, the phase qubit 12 can be arranged as a Josephson junction embedded in a superconducting loop (e.g., an inductor). Thus, the “one” and “zero” quantum states of the phase qubit 12 can differ by the presence or absence of one flux quantum in the inductor of the phase qubit 12 (e.g., having a current difference of approximately 3 μA or less).

The RQL readout system 10 also includes an input stage 14, an output stage 16, and an RQL comparator 18. The input stage 14 is configured to propagate a read pulse RD_PLS that is provided to read the quantum state of the phase qubit 12, such as during a readout operation. As an example, the read pulse RD_PLS can be generated from an external circuit, such as the input stage 14 can include at least one input Josephson junction that can sequentially trigger to propagate the read pulse RD_PLS. Similarly, the output stage 16 is configured to propagate an output pulse OUT that is generated in response to the read pulse RD_PLS in a first quantum state of the phase qubit 12. For example, the output stage 16 can include at least one output Josephson junction that can sequentially trigger to propagate the output pulse OUT to indicate that the phase qubit 12 is in the first quantum state. Conversely, if the phase qubit 12 is in the second quantum state, no output pulse OUT is generated. Therefore, the output pulse OUT is not propagated at the output stage 16 in response to the read pulse RD_PLS when the phase qubit 12 is in the second quantum state.

The RQL comparator 18 is configured to determine the quantum state of the phase qubit 12 in response to the read pulse RD_PLS, and to generate the output pulse OUT if the phase qubit 12 is in the first quantum state. Therefore, the quantum state of the phase qubit 12 can be indicated to other quantum or classical circuits. For example, the output pulse OUT can be implemented for quantum error correction or a variety of other quantum or classical processing applications. In the example of FIG. 1, the phase qubit 12 is demonstrated as coupled to the RQL comparator 18 symmetrically, such that a bias current I_(Q) can flow through the RQL comparator 18. The direction of the flow of the bias current I_(Q) can be based on the quantum state of the phase qubit 12. For example, the phase qubit 12 can be inductively coupled to the RQL comparator 18, such that the bias current I_(Q) can be magnetically induced to flow through the RQL comparator 18. As described herein, “inductive coupling” refers to magnetic coupling between respective inductors, such that a current flow through one inductor induces a current flow through the other inductor based on a magnetic field through a common core of the respective inductors.

In the example of FIG. 1, the RQL comparator 18 includes Josephson junctions 20 that can be arranged symmetrically with respect to the phase qubit 12. As an example, the RQL comparator 18 can include a first of the Josephson junctions 20 and a second of the Josephson junctions 20 through which the portions of the bias current I_(Q) flow and which are each coupled to the output stage 16. The direction of the bias current I_(Q) in the current loop can thus disproportionally affect a threshold of each of the first and second Josephson junctions 20 to selectively trigger one of the first and second Josephson junctions 20 in response to the read pulse RD_PLS. As a result, in the first quantum state of the phase qubit 12, the first Josephson junction can trigger to provide the output pulse OUT. As an example, the bias current I_(Q) flowing through the first of the Josephson junctions 20 can be added to the read pulse RD_PLS to exceed the threshold of the first of the Josephson junctions 20, thus triggering the first of the Josephson junctions 20 to generate the output pulse OUT. Conversely, in the second quantum state of the phase qubit 12, the second Josephson junction can trigger to prevent the output pulse OUT from propagating on the output stage 16. As an example, the bias current I_(Q) flowing through the first of the Josephson junctions 20 can be subtracted from the read pulse RD_PLS to not reach the threshold of the first of the Josephson junctions 20 to not trigger the first of the Josephson junctions 20, but instead triggering the second of the Josephson junctions 20, resulting in no generation of the output pulse OUT.

The RQL readout system 10 also includes an RQL clock 22 that is configured to generate an RQL clock signal CLK. As an example, the RQL clock signal CLK can be a four phase (e.g., quadrature) clock signal, such as having a very high frequency (e.g., tens of gigahertz). In the example of FIG. 1, the RQL clock signal CLK is demonstrated as being provided to the input stage 14, the output stage 16, and the RQL comparator 18. The read pulse RD_PLS, when combined with the RQL clock signal CLK, can exceed the thresholds of at least a portion of the input Josephson junctions in the input stage, the output Josephson junctions in the output stage, and the Josephson junctions 20 in the RQL comparator 18 to facilitate triggering of the respective Josephson junctions. For example, the read pulse can be provided at a first cycle (e.g., positive cycle) of the RQL clock signal CLK, and during a second cycle (e.g., negative cycle) of the RQL clock signal CLK, a negative pulse can be provided to reset the respective Josephson junctions. As an example, the RQL clock signal CLK can be provided to RQL comparator 18 in a common-mode connection, such that the Josephson junctions 20 are arranged symmetrically with respect to the RQL clock signal CLK. Therefore, based on the balanced connection of the RQL clock signal CLK in the input stage 14, the output stage 16, and the RQL comparator 18, the RQL clock signal CLK and associated shunt resistors do not imbalance the bias current I_(Q). Additionally, because of the inductive coupling of the phase qubit 12 to the RQL comparator 18, the RQL clock signal CLK is effectively isolated from the phase qubit 12.

The RQL readout system 10 therefore provides a manner for reading the quantum state of the phase qubit 12 in a very rapid manner with substantially mitigated decoherence. Because the RQL readout system 10 implements RQL-based quantum logic, the RQL readout system 10 can be configured to provide readout of the quantum state of the phase qubit 12 at very rapid data rates (e.g., reading the quantum state in less than one nanosecond) with substantially minimal losses and generated heat (e.g., operating at temperatures of approximately 20 mK without causing local heating). Additionally, based on the balanced fabrication of the RQL comparator 18 with respect to the arrangement of the Josephson junctions 20 and the respective interaction with the bias current I_(Q), the RQL readout system 10 can distinguish current amplitude changes of, for example, less than approximately 3 μA, and can thus achieve a very high sensitivity. Furthermore, based on the isolation of the phase qubit 12 from the RQL clock signal CLK and based on the balanced arrangement of the RQL clock signal CLK, the RQL readout system 10 can substantially mitigate decoherence of the quantum state of the phase qubit 12.

It is to be understood that the RQL readout system 10 is demonstrated simplistically, such that the RQL readout system 10 can include one or more circuit devices that interconnect and/or are included in the components of the RQL readout system 10. For example, as described herein, the term “coupled” can refer to electrical coupling through one or more circuit devices (e.g., an inductor) in the associated quantum circuit, such that current and/or current pulses can flow between coupled components through one or more inductors or other devices. Therefore, coupling between devices in the RQL readout system 10 can include coupling through inductors and/or other circuit devices.

FIG. 2 illustrates an example of an RQL readout circuit 50. The RQL readout circuit 50 can correspond to the RQL readout system 10 in the example of FIG. 1. Therefore, the RQL readout circuit 50 can be implemented in a variety of quantum and classical computing environments to read a quantum state from a phase qubit 52. As an example, at least a portion of the RQL readout circuit 50 can be implemented on an IC in a superconducting environment (e.g., at a very low temperature).

The RQL readout circuit 50 also includes an input stage 54, an output stage 56, and an RQL comparator 58. The input stage 54 is configured to propagate a read pulse RD_PLS that is provided to read the quantum state of the phase qubit 52, such as during a readout operation. As an example, the read pulse RD_PLS can be generated from an external circuit, such as the input stage 54 can include at least one input Josephson junction that can sequentially trigger to propagate the read pulse RD_PLS. The input stage 54 includes a set of inductors through which the read pulse RD_PLS is provided. In the example of FIG. 2, the input stage 54 includes a first inductor L₁, a second inductor L₂, a third inductor L₃, and a fourth inductor L₄. The input stage 54 also includes a first Josephson junction J₁ that is coupled to the first and second inductors L₁ and L₂ and to a low-voltage rail (e.g., ground), and includes a second Josephson junction J₂ that is coupled to the third and fourth inductors L₃ and L₄ and to the low-voltage rail.

The first and second Josephson junctions J₁ and J₂ are configured to sequentially trigger to propagate the read pulse RD_PLS along the input stage 54 via the second, third, and fourth inductors L₂, L₃, and L₄. In the example of FIG. 2, the RQL readout circuit 50 includes an RQL clock 60 that is configured to generate the RQL clock signal CLK that is provided to the input stage 54 between the second and third inductors L₂ and L₃ via an inductive coupling 61 (e.g., a transformer) and an inductor L_(C1). The RQL clock signal CLK thus facilitates triggering of the first and second Josephson junctions J₁ and J₂. As an example, the read pulse RD_PLS can be provided substantially concurrently with a positive cycle of the RQL clock signal CLK, such that the first and second Josephson junctions J₁ and J₂ can have sufficient current to trigger in response to the read pulse RD_PLS. In the example of FIG. 2, the RQL clock signal CLK is shunted to ground via an inductor L_(C2) and a resistor R₁. In the arrangement of the input stage 54, the resistor R₁ is also provided as a common shunting resistor for the first and second Josephson junctions J₁ and J₂. As a result, the inductor L₃ can provide additional inductive isolation of the resistor R₁ with respect to the phase qubit 52, such as to further mitigate the effects of resistor noise on the qubit coherence time of the phase qubit 52.

The RQL comparator 58 is configured to determine the quantum state of the phase qubit 52 in response to the read pulse RD_PLS, and to generate the output pulse OUT if the phase qubit 52 is in the first quantum state. In the example of FIG. 2, the inductor L_(PQ1) of the phase qubit 52 is demonstrated as inductively coupled to an inductor L_(PQ2) to induce the current I_(Q) based on the quantum state of the phase qubit 52 through the inductor L_(PQ2) and through a transformer 62 that is arranged as an inductive common-mode choke (e.g., to substantially mitigate an inductance acting upon the current I_(Q)). In the example of FIG. 2, the current I_(Q) is demonstrated as currents I_(Q1) and I_(Q2) flowing in opposite directions. However, it is to be understood that the current I_(Q1) and I_(Q2) correspond to the current I_(Q) flowing in opposite directions depending on the quantum state of the phase qubit 52. For example, the current I_(Q1) can correspond to the current I_(Q) flowing in a first direction based on the phase qubit 52 having a first quantum state, and the current I_(Q2) can correspond to the current I_(Q) flowing in a second direction based on the phase qubit 54 having a second quantum state. Therefore, the currents I_(Q1) and I_(Q2) are substantially equal in magnitude and direction based on the quantum state of the phase qubit 52.

The phase qubit 52 is inductively coupled to the RQL comparator 58 via the inductor L_(PQ2), the transformer 62, an inductor L_(I1) that is coupled to the input stage 54, and an inductor L_(O1) that is coupled to the output stage 54. The RQL comparator 58 includes Josephson junctions J₃ and J₄ that are each coupled to the inductor L_(I1) at a node 64, and Josephson junctions J₅ and J₆ that are each coupled to the inductor L_(O1) at a node 66. The Josephson junctions J₄ and J₅ are coupled via inductors L_(RQ1) and L_(RQ2), and the Josephson junctions J₃ and J₆ are coupled to ground. Additionally, the RQL clock signal CLK is provided in a common-mode manner between the inductors L_(RQ1) and L_(RQ2) via an inductive coupling 68 (e.g., a transformer) and a pair of inductors L_(C3) and L_(C4), with a shunt resistor R₂ interconnecting the inductors L_(C3) and L_(C4) to ground. Therefore, the phase qubit 52 is symmetrically coupled to the RQL comparator 58, and the RQL comparator 58 is symmetric with respect to the circuit components therein. Similar to as described previously, the RQL clock signal CLK facilitates triggering of the Josephson junctions therein. As an example, the read pulse RD_PLS can be provided substantially concurrently with a positive cycle of the RQL clock signal CLK, such that the Josephson junction J₃ can propagate the read pulse RD_PLS to the Josephson junctions J₅ and J₆, and one of the Josephson junctions J₅ and J₆ can have sufficient current to trigger in response to the read pulse RD_PLS. In the example of FIG. 2, the resistor R₂ is symmetrically arranged with respect to the phase qubit 52 as a common shunt resistor with respect to the Josephson junctions J₃, J₄, J₅, and J₆, as opposed to typical superconducting circuit applications in which shunt resistors are provided in parallel with each respective Josephson junction of the given circuit. Accordingly, the symmetrical common shunt arrangement of the resistor R₂ substantially mitigates the presence of noise that could degrade a coherence time of the phase qubit 52.

As described previously, the direction of the flow of the bias current I_(Q) can be based on the quantum state of the phase qubit 52, and thus flows through the inductor L_(O1) and into the node 66 as the current I_(Q1), or flows out from the node 66 and through inductor L_(O1) as the current I_(Q2). In the example of FIG. 2, the current I_(Q1) is demonstrated as splitting at the node 66, with a first portion of the current I_(Q1) (demonstrated as a current I_(J51)) flowing through the Josephson junction J₅ to the inductor L_(RQ2) and a second portion of the current I_(Q1) (demonstrated as a current I_(J61)) flowing through the Josephson junction J₆ to ground. Similarly, the current I_(Q2) is demonstrated as joining at the node 66, with a first portion of the current I_(Q2) (demonstrated as a current I_(J52)) flowing through the Josephson junction J₅ to the node 66 and a second portion of the current I_(Q2) (demonstrated as a current I_(J62)) flowing through the Josephson junction J₆ to the node 66 from ground. Therefore, currents I_(J51) and I_(J61) correspond to the current I_(Q1), and thus the first quantum state of the phase qubit 52, and the currents I_(J52) and I_(J62) correspond to the current I_(Q2), and thus the second quantum state of the phase qubit 52. Accordingly, the currents I_(J51) and I_(J61) and the currents I_(J52) and I_(J62) change the relative threshold of the Josephson junctions J₅ and J₆ with respect to the read pulse RD_PLS.

During a read operation, the read pulse RD_PLS propagates through the input stage 54 (e.g., through the inductors L₁, L₂, L₃, and L₄ via the Josephson junctions J₁ and J₂) and through the inductor L_(I1) to trigger the Josephson junction J₃. The read pulse RD_PLS thus gets propagated through inductors L_(RQ1) and L_(RQ2) to the Josephson junctions J₅ and J₆. Thus, the bias current I_(Q) is either added to or subtracted from the read pulse RD_PLS with respect to Josephson junctions J₅ and J₆ based on the quantum state of the phase qubit 52.

For example, in the first quantum state of the phase qubit 52, the current I_(Q1) flows into the node 66, and thus the current I_(J51) flows from the node 66 through the Josephson junction J₅ and the current I_(J61) flows from the node 66 through the Josephson junction J₆. Because the current I_(J51) flows oppositely with respect to the read pulse RD_PLS propagated from the Josephson junction J₃ and through the inductors L_(RQ1) and L_(RQ2), the current I_(J51) is subtracted from the read pulse RD_PLS (e.g., increases the threshold of the Josephson junction J₅). Thus, Josephson junction J₅ does not trigger in the first quantum state of the phase qubit 52. However, because the current I_(J61) flows in the same direction as the read pulse RD_PLS propagated from the Josephson junction J₃ and through the inductors L_(RQ1) and L_(RQ2), the current I_(J61) is added to the read pulse RD_PLS (e.g., decreases the threshold of the Josephson junction J₆). Thus, Josephson junction J₆ triggers in the first quantum state of the phase qubit 52.

As another example, in the second quantum state of the phase qubit 52, the current I_(Q2) flows from the node 66, and thus the current I_(J52) flows into the node 66 through the Josephson junction J₅ and the current I_(J62) flows into the node 66 through the Josephson junction J₆. Because the current I_(J52) flows in the same direction as the read pulse RD_PLS propagated from the Josephson junction J₃ and through the inductors L_(RQ1) and L_(RQ2), the current J_(J52) is added to the read pulse RD_PLS (e.g., decreases the threshold of the Josephson junction J₅). Thus, Josephson junction J₅ triggers in the second quantum state of the phase qubit 52. However, because the current I_(J62) flows in the opposite direction as the read pulse RD_PLS propagated from the Josephson junction J₃ and through the inductors L_(RQ1) and L_(RQ2), the current J_(J62) is subtracted from the read pulse RD_PLS (e.g., increases the threshold of the Josephson junction J₆). Thus, Josephson junction J₆ does not trigger in the second quantum state of the phase qubit 52.

The output stage 56 is configured to propagate the output pulse OUT that is generated in response to the triggering of the Josephson junction J₆, and thus in the first quantum state of the phase qubit 52. The output stage 56 is demonstrated in the example of FIG. 2 as being arranged substantially similar (e.g., symmetrical) with respect to the input stage 54. In the example of FIG. 2, the output stage 56 includes a set of inductors through which the output pulse OUT propagates from the Josephson junction J₆ and through the inductor L_(O1). In the example of FIG. 2, the output stage 56 includes a first inductor L₅, a second inductor L₆, a third inductor L₇, and a fourth inductor L₈. The output stage 56 also includes a first Josephson junction J₇ that is coupled to the third and fourth inductors L₇ and L₈ and to ground, and includes a second Josephson junction J₈ that is coupled to the first and second inductors L₅ and L₆ and to ground. The first and second Josephson junctions J₇ and J₈ are configured to sequentially trigger to propagate the output pulse OUT along the output stage 56 via the second, third, and fourth inductors L₆, L₇, and L₈. In the example of FIG. 2, the RQL clock signal CLK is provided to the output stage 56 between the second and third inductors L₆ and L₇ via an inductive coupling 70 (e.g., a transformer) and an inductor L_(C5), and is shunted to ground via an inductor L_(C6) and a resistor R₃. In the arrangement of the output stage 56, the resistor R₃ is also provided as a common shunting resistor for the first and second Josephson junctions J₇ and J₈. As a result, the inductor L₆ can provide additional inductive isolation of the resistor R₃ with respect to the phase qubit 52, such as to further mitigate the effects of resistor noise on the qubit coherence time of the phase qubit 52.

The RQL clock signal CLK thus facilitates triggering of the first and second Josephson junctions J₇ and J₈. Accordingly, in response to the triggering of the Josephson junction J₆ in response to the read pulse RD_PLS in the first quantum state of the phase qubit 52, the output pulse OUT is provided at the output stage 56 to indicate the first quantum state of the phase qubit 52. Conversely, in response to the triggering of the Josephson junction J₅ instead of the Josephson junction J₆ in response to the read pulse RD_PLS in the second quantum state of the phase qubit 52, the output pulse OUT is not provided at the output stage 56 to indicate the second quantum state of the phase qubit 52.

It is to be understood that the RQL readout circuit 50 is not limited to the example of FIG. 2. As an example, the input stage 54 and the output stage 56 are not limited to as demonstrated in the example of FIG. 2, but can include additional circuit components, such as one or more additional Josephson junctions and/or one or more additional inductors. Additionally, the readout operation described herein is not limited to readout of a phase qubit 52, but a variety of other qubits or circuit devices having logic or quantum states that are based on a current flow direction can be implemented in the RQL readout circuit 50. Accordingly, the RQL readout circuit 50 can be configured in a variety of ways.

FIG. 3 illustrates an example of a timing diagram 100. The timing diagram 100 can correspond to the timing of the RQL readout circuit 50. Therefore, reference is to be made to the example of FIG. 2 in the following description of the example of FIG. 3. The timing diagram 100 demonstrates the current I_(Q1), such that a positive magnitude of the current I_(Q1) demonstrates that the current I_(Q) is provided as the current I_(Q1) in the first quantum state of the phase qubit 52 and a negative magnitude of the current I_(Q1) demonstrates that the current I_(Q) is provided as the current I_(Q2) in the second quantum state of the phase qubit 52. The timing diagram 100 also demonstrates the read pulse RD_PLS, the current I_(J51), the current I_(J61), and the output pulse OUT. The current I_(J51) is demonstrated as having a negative magnitude when the current I_(Q) is provided as the current I_(Q2) in the second quantum state, and is demonstrated as having a positive magnitude to correspond to the current I_(J52) when the current I_(Q) is provided as the current I_(Q1) in the first quantum state. Similarly, the current I_(J61) is likewise demonstrated as having a negative magnitude when the current I_(Q) is provided as the current I_(Q1) in the second quantum state, and is demonstrated as having a positive magnitude to correspond to the current I_(J62) when the current I_(Q) is provided as the current I_(Q2) in the first quantum state.

At a time T₀, the current I_(Q1) is negative, such that the phase qubit 52 is in the second quantum state. The read pulse RD_PLS is provided at the input stage 54, such as concurrently with a positive cycle of the RQL clock signal CLK. The current I_(J51) is negative at the time T₀, and thus is added to the read pulse RD_PLS with respect to the Josephson junction J₅. The current I_(J61) is also negative, and thus is subtracted from the read pulse RD_PLS with respect to the Josephson junction J₆. Therefore, the Josephson junction J₅ triggers, and the Josephson junction J₆ does not trigger. As a result, the output pulse OUT is not generated and propagated along the output stage 56. Accordingly, the output stage 56 indicates that the phase qubit 52 is in the second quantum state. At a time T₁, the read pulse RD_PLS is provided as a negative pulse, such as substantially concurrently with a negative cycle of the RQL clock signal CLK, to reset the Josephson junctions of the RQL readout circuit 50. At a time T₂, the read pulse RD_PLS is provided again, resulting in an indication of the phase qubit 52 being in the second quantum state again, similar to as described previously. At a time T₃, the read pulse RD_PLS is again provided as a negative pulse to reset the Josephson junctions of the RQL readout circuit 50.

At a time T₄, the current I_(Q1) is positive, such that the phase qubit 52 is in the first quantum state. The read pulse RD_PLS is provided at the input stage 54, the current I_(J51) is positive, and thus is subtracted from the read pulse RD_PLS with respect to the Josephson junction J₅. The current I_(J61) is likewise positive, and thus is added to the read pulse RD_PLS with respect to the Josephson junction J₆. Therefore, the Josephson junction J₅ does not trigger, and the Josephson junction J₆ triggers. As a result, the output pulse OUT is generated and propagates along the output stage 56. Accordingly, the output stage 56 indicates that the phase qubit 52 is in the first quantum state. At a time T₅, the read pulse RD_PLS is provided as a negative pulse to reset the Josephson junctions of the RQL readout circuit 50. At a time T₆, the read pulse RD_PLS is provided again, resulting in an indication of the phase qubit 52 being in the first quantum state again, similar to as described previously. At a time T₇, the read pulse RD_PLS is again provided as a negative pulse to reset the Josephson junctions of the RQL readout circuit 50.

In view of the foregoing structural and functional features described above, a methodology in accordance with various aspects of the present invention will be better appreciated with reference to FIG. 4. While, for purposes of simplicity of explanation, the methodology of FIG. 4 is shown and described as executing serially, it is to be understood and appreciated that the present invention is not limited by the illustrated order, as some aspects could, in accordance with the present invention, occur in different orders and/or concurrently with other aspects from that shown and described herein. Moreover, not all illustrated features may be required to implement a methodology in accordance with an aspect of the present invention.

FIG. 4 illustrates an example of a method 150 for reading a quantum state of a phase qubit (e.g., the phase qubit 12). At 152, a bias current (e.g., the bias current I_(Q)) is provided through a first Josephson junction (e.g., the Josephson junction J₆) and a second Josephson junction (e.g., the Josephson junction J₅) in a first direction (e.g., the bias current I_(Q1)) in a first quantum state of the phase qubit and in a second direction (e.g., the bias current I_(Q2)) in a second quantum state of the phase qubit. At 154, a read pulse (e.g., the read pulse RD_PLS) is applied at a first cycle of an RQL clock signal (e.g., the RQL clock signal CLK), the read pulse being propagated on an input stage (e.g., the input stage 14) to the first and second Josephson junctions. At 156, the phase qubit is determined to be in the first quantum state based on receiving an output pulse (e.g., the output pulse OUT) at an output stage (e.g., the output stage 16) in response to the read pulse the first Josephson junction triggering based on the bias current being provided in the first direction and the read pulse, or in the second quantum state based on not receiving output pulse at the output stage in response to the second Josephson junction triggering based on the bias current being provided in the second direction and the read pulse.

What have been described above are examples of the invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the invention are possible. Accordingly, the invention is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. 

What is claimed is:
 1. A readout system configured to read a quantum state of a qubit, the readout system comprising: an input stage on which a read pulse is provided; an output stage; and a comparator circuit coupled to the qubit and configured to provide an output pulse on the output stage in response to the read pulse to indicate a first quantum state of the qubit, and to not provide the output pulse on the output stage in response to the read pulse to indicate a second quantum state of the qubit.
 2. The system of claim 1, wherein the comparator circuit is configured to provide output pulse on the output stage in response to the read pulse to indicate the first quantum state of the qubit and to not provide the output pulse on the output stage in response to the read pulse to indicate the second quantum state of the qubit based on a bias current provided to the comparator circuit having a characteristic that corresponds to one of the first and second quantum states of the qubit.
 3. The system of claim 2, wherein the characteristic of the bias current is current direction, such that the bias current has a first current direction based on the first quantum state of the qubit and has a second current direction opposite the first current direction based on the second quantum state of the qubit.
 4. The system of claim 2, wherein the comparator circuit comprises a first Josephson junction and a second Josephson junction that are coupled to the output stage, such that the read pulse triggers the first Josephson junction to provide the output pulse on the output stage in response to the bias current based on the first quantum state of the qubit, and such that the read pulse triggers the second Josephson junction to provide no output pulse on the output stage in response to the bias current based on the second quantum state of the qubit.
 5. The system of claim 1, further comprising an RQL clock configured to generate an RQL clock signal configured to propagate the read pulse through the input stage, to activate the comparator circuit in response to the read pulse, and to propagate the output pulse through the output stage in response to indication of the first quantum state of the qubit.
 6. The system of claim 5, wherein the RQL clock signal is provided to the input stage, the output stage, and the comparator circuit in a symmetrical manner, and wherein the qubit is substantially isolated from the RQL clock signal via at least one inductive coupling.
 7. The system of claim 1, wherein the qubit is inductively coupled to the comparator circuit to generate a bias current that flows through each of a first Josephson junction associated with the comparator circuit and a second Josephson junction associated with the comparator circuit in a first direction in the first quantum state and in a second direction in the second quantum state, the bias current being provided to change a relative threshold associated with each of the first and second Josephson junctions with respect to the read pulse.
 8. The system of claim 7, wherein the bias current through the first Josephson junction in the first direction is added to the read pulse to trigger the first Josephson junction to generate the output pulse and the bias current through the second Josephson junction in the first direction is subtracted from the read pulse to prevent the second Josephson junction from triggering, and wherein the bias current through the first Josephson junction in the second direction is subtracted from the read pulse to prevent triggering of the first Josephson junction and the bias current through the second Josephson junction in the second direction is added to the read pulse to trigger the second Josephson junction.
 9. The system of claim 7, wherein the comparator circuit further comprises a third Josephson junction and a fourth Josephson junction, wherein the first and second Josephson junctions are coupled to the output stage and wherein the third and fourth Josephson junctions are coupled to the input stage to substantially balance the comparator circuit, and wherein the qubit is inductively coupled to the input stage and to the output stage to generate the bias current through the first, second, third, and fourth Josephson junctions in a first direction in the first quantum state and in a second direction in the second quantum state.
 10. The system of claim 1, wherein the comparator circuit comprises a shunt resistor that symmetrically interconnects an RQL clock signal and the comparator circuit, such that the shunt resistor is symmetrically arranged with respect to the qubit.
 11. A method for reading a quantum state of a phase qubit, the method comprising: providing a bias current in a first direction in response to a first quantum state of the phase qubit and in a second direction in response to a second quantum state of the phase qubit; applying a read pulse on an input stage; providing an output pulse from a comparator circuit to an output stage in response to the read pulse based on the bias current being provided in the first direction; and not providing the output pulse from the comparator circuit to the output stage in response to the read pulse based on the bias current being provided in the second direction.
 12. The method of claim 11, wherein providing the bias current comprises providing the bias current through each of a first Josephson junction and a second Josephson junction in first respective directions in a first quantum state of the phase qubit and in second respective directions in a second quantum state of the phase qubit, wherein determining that the phase qubit is in the first quantum state comprises determining that the phase qubit is in the first quantum state in response to the first Josephson junction triggering based on the bias current being provided in the first direction through the first Josephson junction, and wherein determining that the phase qubit is in the second quantum state comprises determining that the phase qubit is in the second quantum state in response to the second Josephson junction triggering based on the bias current being provided in the second direction through the second Josephson junction.
 13. The method of claim 12, wherein providing the bias current comprises: providing the bias current in the first direction to decrease a threshold of the first Josephson junction relative to the second Josephson junction; and providing the bias current in the second direction to decrease a threshold of the second Josephson junction relative to the first Josephson junction.
 14. The method of claim 11, further comprising: providing an RQL clock signal to the input stage to facilitate triggering of at least one input Josephson junction to propagate the read pulse; providing the RQL clock signal to the output stage to facilitate triggering of at least one output Josephson junction to propagate the output pulse in response to the read pulse in the first quantum state of the phase qubit; and providing the RQL clock signal to the RQL comparator to facilitate triggering of at least one Josephson junction in response to the read pulse to provide or to not provide the output pulse based on the respective one of the first and second quantum states of the phase qubit.
 15. The method of claim 11, wherein providing the bias current comprises providing the bias current based on an inductive coupling of the phase qubit to the comparator circuit.
 16. A reciprocal quantum logic (RQL) readout system configured to read a quantum state of a qubit, the RQL readout system comprising: an input stage on which a read pulse is provided in response to an RQL clock signal; an output stage configured to propagate an output pulse in response to the RQL clock signal; and an RQL comparator coupled to the qubit and configured to provide an output pulse on the output stage based on triggering a first Josephson junction in response to the read pulse and the RQL clock signal to indicate a first quantum state of the qubit, and to not provide the output pulse on the output stage based on triggering a second Josephson junction in response to the read pulse and the RQL clock signal to indicate a second quantum state of the qubit.
 17. The system of claim 16, wherein the qubit inductively generates a bias current through the first and second Josephson junctions in a first direction in the first quantum state and in a second direction in the second quantum state, the bias current being provided to change the relative threshold associated with each of the first and second Josephson junctions.
 18. The system of claim 17, wherein the bias current through the first Josephson junction in the first direction is added to the read pulse to trigger the first Josephson junction to generate the output pulse and the bias current through the second Josephson junction in the first direction is subtracted from the read pulse to prevent the second Josephson junction from triggering, and wherein the bias current through the first Josephson junction in the second direction is subtracted from the read pulse to prevent triggering of the first Josephson junction and the bias current through the second Josephson junction in the second direction is added to the read pulse to trigger the second Josephson junction.
 19. The system of claim 16, wherein the RQL comparator further comprises a third Josephson junction and a fourth Josephson junction, wherein the first and second Josephson junctions are coupled to the output stage and wherein the third and fourth Josephson junctions are coupled to the input stage.
 20. The system of claim 16, wherein the RQL clock signal is provided symmetrically with respect to the qubit, wherein the read pulse is substantially synchronized with the first cycle of the RQL clock signal. 